The application of coatings, such as resin coatings and foam coatings to substrates is known in the papermaking art. For instance, it is known to apply a photosensitive resin to a foraminous member in a preselected pattern for use in a papermaking operation. It is also known to provide papermaking press fabrics with a coating, such as a foam coating, to achieve a controlled void volume and permeability. The following documents describe the use of resins, fillers, foams, layered constructions, or other coatings in making papermaking equipment: U.S. Pat. No. 3,549,742 issued Dec. 22, 1970 to Benz; U.S. Pat. No. 4,446,187 to Eklund; U.S. Patent 4,514,345 issued Apr. 30, 1985 to Johnson et al.; U.S. Pat. No. 4,637,859 issued Jan. 20, 1987 to Trokhan; U.S. Pat. No. 4,795,480 issued Jan. 3, 1989 to Boyer et al.; U.S. Pat. No. 5,098,522 issued Mar. 24, 1992 to Smurkoski et al.; U.S. Pat. No. 5,346,567 issued Sep. 13, 1994 to Barnewall; U.S. Pat. No. 5,334,289 issued Aug. 2, 1994 to Trokhan et al.; and PCT Publication Number WO 91/14558 published Oct. 3, 1991 in the name of Sayers et al. and assigned to the SCAPA Group.
It is also known to impregnate textile fabrics, such as needled fiber mats and felt material, with resins and filler materials. The following documents describe the use of resins and/or fillers in fabrics: U.S. Pat. No. 4,250,172 to Mutzenberg et al; U.S. Pat. No. 4,390,574 to Wood; U.S. Pat. No. 4,464,432 to Dost et al.; U.S. Pat. No. 5,217,799 to Sumii et al.; U.S. Pat. No. 5,236,778 to Landis et al.; and Reissue Pat. No. 32,713 reissued Jul. 12, 1988 to Woo.
After curing a portion of the resin on a substrate to form a papermaking apparatus, it is desirable to remove uncured resin from the substrate. Removal of uncured resin from the substrate is important so that the resulting papermaking apparatus has the desired characteristics for its particular papermaking application. Such characteristics can include, but are not limited to, flexibility of the apparatus, compressibility of the apparatus, durability of the apparatus, air permeability through the apparatus, and water permeability through the apparatus. Removal of uncured resin is especially important in a papermaking apparatus having a patterned resin surface with openings through which air and/or water is conveyed during formation or drying of the paper web. Uncured resin remaining in the substrate can reduce the permeability of the substrate, and thereby reduce flow through the openings in the patterned resin surface.
One method for removing uncured resin includes washing uncured resin from the substrate. For instance, above referenced U.S. Pat. No. 4,514,345 discloses washing uncured resin from a foraminous member formed of woven filaments, followed by vacuuming of residual wash liquid and uncured liquid from the foraminous member. However, washing and vacuuming, alone, can be ineffective in removing all uncured resin.
A felt or open celled foam substrate can have a large number of relatively small, internal void cavities which can trap uncured resin. Such trapped uncured resin can degrade the performance of the papermaking apparatus, as described above. In addition, such trapped resin is essentially wasted, adding to the cost of the resin casting process. Removal of the trapped resin by increasing the number of washing and vacuum cycles also increases the cost of the process.
Moreover, in some applications it may be desirable to control the depth of penetration of the resin into the substrate. For instance, it may be desirable to have the cured resin layer penetrate a predetermined portion of the thickness of the substrate, so as to provide acceptable bonding of the resin to the substrate, while maintaining the flexibility of the substrate and the permeability of the substrate to air and water.
U.S. Pat. No. 3,549,742 issued Dec. 22, 1970 to Benz discloses inserting filling material into apertures in a drainage member which will ultimately be open for drainage, after which a settable material is inserted into the remaining apertures of the drainage member in the predetermined areas in which flow of liquid through the drainage member is to be prevented. The settable material is fixed or cured, after which the filling material is removed from the drainage member. Benz has the disadvantage that the filler material is arranged in a predetermined pattern prior to application to the drainage member, and the filler material must be pressed into the drainage member such that predetermined areas of the drainage member are left free of the filling material. Accordingly, the pattern in which the settable material can be fixed to the drainage member is limited by the predetermined areas of the drainage member left free of the filling material.
Also, Benz uses pressure to mechanically force the filler material into the drainage member. Pressing a filling material into a substrate can suffer from the disadvantage that, if the substrate has many small internal voids, and is relatively compressible, applying pressure to the substrate can collapse the substrate or close some of the voids in the substrate, thereby making penetration of the substrate by the filler material difficult.
In addition, pressing a filling material into a felt layer can result in the filling material flowing laterally into areas of the felt which are meant to be left open for the settable material. Therefore, the method disclosed by Benz is undesirable for use in applying a curable resin to a felt layer.
U.S. application Ser. No. 08/388,948 of McFarland et al. discloses a method of applying a curable resin to a substrate to form a papermaking apparatus. The method of McFarland et al. reduces the amount of uncured photosensitive resin that is required to be removed from a paper web patterning apparatus suitable for making paper having visually discernible patterns. The teachings of McFarland are herein incorporated by reference.
McFarland et al. discloses a method of applying the filler material to the substrate to occupy at least some of the voids in the substrate, applying a curable resin to the substrate, curing at least some of the resin to provide a resin layer on the substrate; and then removing at least some of the filler material from the substrate, to leave the substrate fluid permeable and suitable as a dewatering felt.
The method of McFarland et al. is successful in forming the papermaking apparatus comprising a felt or open celled foam substrate, but has one significant drawback. Upon application of the curable resin to the surface of the substrate air can be trapped between the curable resin layer and the void-filled substrate. The filler material in the voids of the substrate prevents the trapped air from escaping. The air then migrates, or floats, into the resin, forming bubbles in the resin. Subsequent curing of the resin permanently traps the air bubbles, compromising the integrity of the cured resin structure.
The presence of air bubbles in the cured resin affects the ultimate durability of the papermaking apparatus. By making the papermaking apparatus more durable, costs are lowered and a better paper product can be more consistently produced. Elimination of air bubbles in the resin is desirable not only for the specific impact on durability, but also because they present an uncontrolled variable in the process of making a papermaking apparatus.
Accordingly, one object of the invention is to provide a method for reducing the amount of trapped air bubbles in a curable resin for a paper web patterning apparatus suitable for making paper having visually discernible patterns.
Another object is to provide a method for forming a web patterning apparatus having a dewatering felt layer and a substantially bubble-free patterned photosensitive resin layer which penetrates a surface of the felt layer and extends from the surface of the felt layer.